Traveling wave interaction device



y 1965 R. H. PANTFZUM 3,183,399

TRAVELING WAVE INTEIH-{ETION DEVICE Filed May 31 1960 zwmroa RICHARD H. PANT ELL BY United States Patent 3,183,399 TRAVELHJG WAVE INTERACTION DEVICE Richard H. Pantell, Pale Alto, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed May 31, 1960, Ser. No. 32,762 11 Claims. (Cl. 315-4) The present invention relates to electron tubes, and more particularly, to a traveling wave interaction device capable of effective operation as an amplifier or oscillator at frequencies in the neighborhood of 300 kilomegacycles, or in other words, in the millimeter wavelength region.

When the desirability of producing and amplifying radio requency energy in the mircowave region, or in other words, at frequencies in the neighborhood of l kmc. (kilomegacycles) first arose, the limitations of triodes and other conventional low frequency electron tubes became apparent. Briefly, such limitations were a result of the finite spacing between the electrodes of such low frequency tubes; the spacing, for example, between the cathode and grid of the triode was so great that the time for transit of electrons consumed an appreciable part of a cycle at such higher frequencies and therefore restricted the use of such tubes to lower frequencies. Velocity modulation tubes, first the klystron and more recently the traveling wave tube, enabled the circumvention of the transit-time problem and very sucessful results in the microwave region up to approximately 30 kmc. However, as yet higher frequencies are desired, limitations in both the klystron and traveling wave tube also appear, such limitations again being the result of the inherent structural characteristics of these tubes. In the case of the klystron, as the frequency increases, the size of the resonant cavity or cavities decreases so that at 30 13110., the construction of such cavities becomes a tedious jewelers job, and at 100 kmc. an impossible task. Somewhat similary, in the case of the traveling wave tube, the so-called slow-wave" structure which enables propagation of the radio frequency energy with a phase velocity commensurate with the practically obtainable velocities of electrons in a beam is also inversely proportional in physical size to the operating frequency so that the construction of the helix or other slowwave structure is again a tedious, difficult problem at 30 kmc. or more. Furthermore, in view of the fact that the radio frequency fields supported by such a structure have field strengths which decay exponentially in magnitude away from the structure, the electron beam must pass extremely close to such structure at very high frequencies in order to obtain good interaction and effective amplification or oscillation. Thus beam interception with the rather small, delicate structure on which the slow-wave is propagated, can well result in damage or actual destruction of such structure, particularly when relatively high power operation is experienced.

Accordingly, it is a general object of the present invention to provide a traveling wave interaction device that is effectively operable in the millimeter wave region, or in other words, at frequencies in the neighborhood of 300 kmc. (kilomegacycles).

More particularly, it is a feature of the invention to provide a traveling wave interaction device in which the radio frequency energy can be propagated as a fast-wave with a phase velocity at or greater than the velocity of light.

It is a further feature of the invention to provide a traveling wave interaction device wherein the electron beam can pass through the structure in a position substantially removed from the physical elements thereof so that beam interception is eliminated yet the radio frequency fields are such that the interaction in a region of high field strength is obtainable.

It is another feature of the invention to provide a traveling wave interaction device that is capable of operation over a relatively broad band of frequencies.

Yet a further feature of the invention is the provision of a traveling wave interaction device which is tunable in a relatively simple fashion through the mere expedient of variation of a magnetic field over a two-to-one frequency range.

Still a further feature of the invention relates to the provision of a traveling wave interaction device that can utilize the fast wave propagation structures such as those normally employed for the transmission of high frequency electromagnetic energy as the interaction structure wherefore energy can be coupled into and out of the interaction section of the device through components substantially identical with such interaction section so that coupling and matching problems are reduced to a substantial nullity.

A correlated feature of the invention is the provision of a traveling wave interaction device in which standard unloaded waveguide or a coaxial or multiwire transmission line can be employed for the interaction section of the device thus obviating the necessity for fabrication of a complicated interaction structure.

These as well as other objects and features of the invention will become more apparent from a perusal of the following description of the structure illustrated in the accompanying drawings wherein:

FIG. 1 is a diagrammatic view, partially in central longitudinal section, of a traveling wave interaction device constituting an exemplary embodiment of the invention,

FIG. 2 is a diagrammatic perspective view illustrating the character of interaction between electrons in the beam of the device of FIG. 1 and the transverse radio frequency magnetic field which effects the desired bunching of the electrons in a novel manner, and

FIG. 3 is a similar diagrammatic perspective view illustrating the interaction of the electrons with the transverse radio frequency electric field that results in the amplification of the radio frequency energy.

Generally, in order to obtain amplification of radio frequency energy such as is achieved in a conventional traveling wave tube, it is necessary that the electrons in the beam be bunched and that the bunched electrons move in substantial synchronism with an appropriate phase of the propagated radio frequency energy so that energy is delivered from the electrons to the radio frequency wave. Whereas in the conventional traveling wave tube, such energy interchange is obtained by slowing the propagation of the radio frequency wave so that a longitudinal phase velocity substantially equal to the velocity of the electrons in the beam is obtained, in accordance with the present invention, the radio frequency energy is allowed to propagate with a phase velocity at or greater than the :action region even though the longitudinal velocity of such electrons is much slower than the longitudinal phase velocity of the radio frequency energy.

While radio frequency energy can, in accordance with I the present invention, be propagated through the interaction region as a fast wave by any standard transmission line such as a coaxial line, a two-wire transmission line or an unloaded waveguide, for purposes of explanation, the invention will be described relative to the latter type of fast-wave propagating structure, as illustrated in FIG. 1.

In such FIG. 1, an unloaded waveguide section, indicated at 10, and consisting of the standard rectangular copper tube, is arranged to propagate radio frequency energy with a phase velocity at or greater than the velocity of light and is also arranged to accommodate an electron beam in a manner such that interacting relationship between the beam and the radio frequency fields is set up to secure the desired amplification and/or oscillation. The waveguide section is straight, as shown, and is connected at its opposite ends by conventional flanges to similar waveguide sections 12, 14 which serve to couple radio frequency energy into and out of the interaction [waveguide section 10. The coupling waveguide section 12 to the left in FIG. 1 is curved so that its outer end projects substantially rectangularly from the longitudinal direction of the interaction waveguide section 10 and mounts a conventional waveguide window 16 at its extremity. A circular opening 13 is formed in the side wall of this coupling waveguide section 12 so that an electron beam may pass therethrough and into the interaction waveguide section 10 to traverse the same substantially centrally thereof and eventually pass into the second coupling waveguide section 14 which is slightly bent wherefore its one wall functions'as a collector, as indicated at 20, forthe electrons in the beam. Like the first coupling waveguide section '12, this second coupling waveguide section 14 is also provided with a vacuum-tight window 22 at its outer extremity.

The mentioned electron beam is produced and directed into the interaction section of waveguide 10 from an electron gun, indicated generally at 30. Such electron gun 30 includes an annular cathode 32 supported in alignment with an anode 34 having an annular gap through which the generated electrons may pass in the form of a hollow beam into the interaction waveguide section 10. A relatively small, cylindrical passage 36, merely large enough to accommodate the generated beam, is formed between the anode 34 and the entrance to the waveguide interaction section so that while the beam may pass therethrough, radio frequency energy is precluded entry into the electron gun itself. In the normal fashion, the anode 34 is supplied with a potential sufficie-ntly positive relative to the cathode 32 to produce the desired longitudinal velocity of the electrons in the beam.

Additionally, in accordance with the present invention, the anode 3 4 and associated parts are composed of magnetic material and magnetic energy generated from a coil 38 within the gun 30 is arranged to produce a radial magnetic field across the anode gap through which the electrons pass; rwherefore, in addition to the mentioned longitudinal velocity,'the electrons are given an angular component of motion prior to their entry or injection into the interaction waveguide section 10. The radial magnetic field should be suificiently strong to produce components of motion, are subjected'to a strong mag-' netic field extending longitudinally. of the interaction section and produced by an encompassing solenoid '40. In accordance with known principles, the electrons in-, jected into such longitudinally-extending magnetic field with some initial transverse motion will rotate in such field at an electron-cyclotron resonance frequency which directed to the right at a distance along the waveguide frequency wave than is extracted therefrom.

is directly correlated with the strength of such magnetic field. Since the electrons have both longitudinal and rotative components of motion, they traverse a generally helical path.

Thus, each of the electrons is caused to traverse a periodic trajectory as it moves through the interaction waveguide section 10 and by appropriate adjustment of the strength of the longitudinally-extending magnetic field through the simple expedient of varying the current through the solenoid 49, this trajectory can be controlled so that the desired phase relationship between the clectronsand a radio frequency wave propagated through the interaction section can be achieved. More particularly, the magnetic field is adjusted so that the mentioned electron-cyclotron resonance frequency is substantially equal to the frequency of the propagated radio frequency wave, and under this condition amplification can be obtained as will now be explained.

It is known that amplification of radio frequency energy can be obtained through both forward-Wave and backward-wave interaction since the energy includes wave components which travel in the same direction and in the opposite direction from the direction of actual energy transmission. For a further explanation of such wave components, any standard text dealing with traveling wave tube interaction can be consulted. For the present purposes, it is merely necessary to state that if radio frequency energy is supplied to the waveguide coupling section 12 adjacent the electron gun 30, appropriate adjustment of the magnetic field will result in forwardwave amplification during traversal of the radio frequency energy from the left to the right through the waveguide interaction section 10 and the amplified radio frequency energy will be coupled out through the coupling waveguide section 14 to the right in FIG. 1. On the other hand, if radio frequency energy is supplied to the coupling waveguide section 14 at the collector end of the device, interact-ion of the electrons with a backward-wave" component will result in backwardwave amplification and, in turn, the amplified radio frequency energy will now be removed through the coupling waveguide section 12 adjacent the gun end of the device. Furthermore, the device can be readily arranged for use as a backward wave oscillator through the mere connection to the coupling waveguide section 14 of a suitable resistive load matched to the interaction structure in a known manner. The radio frequency oscillations produced will be removed from the input coupling waveguide 12.

As previously mentioned, in order to obtain amplification or oscillation, it is necessary that a bunching of the electrons be effected so that the electrons will interact with the radio frequency energy in proper phase relationship so that more energy is delivered to the radio In the present device, a novel, predominantly magnetically induced'bunching is achieved in a manner which can best be described by reference to FIG. 2. Insuch FIG. 2,

it is assumed that a hollow beam of electrons c has been injected. into the interaction section 10 with a substantial rotative component of motion due to the lon gitudinal magnetic field produced by the solenoid 40. Furthermore, radio frequency energy has been supplied to the waveguidesection :10, as illustrated in phantom lines in FIG. .2, so that such energy is being conducted in a dominant mode normally referred to as the TE mode. Under such mode condition, the instantaneous .rnagnetic field of the propagated radio frequency energy will appear as the lines H directedto the left at the entrance end' of the waveguide section '10, and such magnetic field lines H will reverse themselves so as to be of precisely one-half a guide wavelength, or in other words, electrical degrees therebeyond.

If it is assumed that individual electrons e are rotating in a clockwise direction as they advance along the waveguide, as indicated by the solid arrows in FIG. 2, an electron 2 moving downwardly through the radio frequency magnetic field H when it is directed to the left, as viewed in FIG. 2, will experience a force component opposite to its general direction of longitudinal movement down the guide while another electron moving upwardly through the same radio frequency magnetic field will experience a force component along its general direction of motion, as indicated by the dotted arrows. To the contrary, electrons 2 moving in the same rotative direction at a distance of 180 electrical degrees down the waveguide will be subjected to opposite forces since the magnetic field is here directed oppositely, or to the right, as viewed in FIG. 2. Thus an electron moving downwardly will be given a force component in its longitudinal direction of motion while an electron moving upwardly is provided with a force component opposite to its general longitudinal direction of motion. While the siutation illustrated in FIG. 2 is an instantaneous one, it will be appreciated that forces will act continuall on the helically moving electrons so as to ultimately effect a change of the beam configuration from that of a hollow cylinder to one of a helical tape. The width of this helical tape constitutes a measure of the degree of bunching; better bunching means a narrower width of the tape. Since, the resultant bunching forces, as indicated by the dotted arrows in FIG. 2, 'on the electrons depend upon the amount of their transverse motion, the initial deflecting radial magnetic field in the electron gun should be relatively strong.

The electrons e having been bunched to a helical tape formation, it will be seen that if the synchronism condition, previously mentioned, exists so that the frequenecy of electron rotation (electron-cyclotron resonance frequency) is equivalent to that of the radio frequency wave, all of the electrons e in properly bunched relation will encounter the same transverse electric field during their traversal of the interaction structure so that maximum transfer of energy from the beam to the radio frequency wave Will be achieved. The mechanism of this transfer can be more fully understood by reference to FIG. 3 wherein again a phantom line illustration of the interaction waveguide section is reproduced together with the existent transverse electric fields E of the TE mode at positions spaced one-half of a guide wavelength along the waveguide so that a complete reversal of the electric fields E is experienced between such spaced positions. An electron or, more precisely, an electron bunch, rotating helically clockwise and down at the position at the entrance end of the waveguide section illustrated in FIG. 3 where the electric field lines E are similarly directed downwardly will be retarded by such field and thus give up energy thereto. Furthermore, one-half cycle later, this same electron or bunch of electrons 2 will be moving upwardly in the interaction structure 10 when the electric fields E are reversed and are also directed upwardly so that again the electrons will be retarded and energy will be transferred therefrom to the radio frequency wave. It will be noted that the electrons, although spaced from the walls of the interaction waveguide, are in regions of high electric field at substantially all times during their traversal so that good interaction and excellent amplification can be achieved.

While it is apparent that the described device circumvents the mentioned limitations of klystrons and conventional traveling Wave tubes so that effective operation at high frequencies is enabled, it is additionall to be noted that, as a result of the good power handling capabilities of the described structure together with its wide band width and ready tunability, a traveling wave interaction device embodying the present invention could also be employed at relatively low frequencies to great advantage.

Various modifications and/or alterations can be made in the structure as described without departing from the spirit of the invention, and the same is to be considered as purely exemplary and not in a limiting sense. The actual scope of the invention is indicated by reference to the appended claims.

What is claimed is:

1. A traveling wave interaction device which comprises, a fast-wave propagating structure, means for injecting an electron beam into said structure generally longitudinally thereof with individual electrons having a transverse component of motion, and means for producing a magnetic field directed generally longitudinally of said propagating structure and having a strength to provide an electron cyclotron resonance frequency substantially equal to the frequency of electromagnetic wave energy propagated by said wave-propagating structure.

2. A traveling wave interaction device according to claim 1 wherein said electron beam injecting means includes means producing a magnetic field directed radially of the beam axis.

3. A traveling wave interaction device according to claim 1 wherein said electron-beam injecting means includes an electron gun for producing a hollow generally cylindical beam.

4. A traveling wave interaction device according to claim 1 wherein said fast-wave propagating structure constitutes a conductive waveguide.

5. A traveling wave interaction device according to claim 1 which comprises means for adjusting the field strength of said magnetic field.

6. A traveling wave interaction device according to claim 1 which comprises means coupled and matched to said fast-wave propagating structure to couple output electromagnetic wave energy therefrom.

7. A traveling wave interaction device according to claim 6 which comprises means coupled and matched to said fast-wave propagating structure at the end remote from said output coupling means for supplying input electromagnetic wave energy thereto.

8. A traveling wave interacting device according to claim 7 wherein said fast-wave propagating structure, and said input and output coupling means constitute sections of conductive waveguide of like cross-sectional dimensions.

9. A traveling wave interaction device which comprises, means for generating an electron beam of generally cylindrical cross-section, means for effecting a rotative component of motion of the electrons in the beam about the beam axis, a fast-wave propagating structure encompassing said electron beam and extending longitudinally thereof whereby electromagnetic energy can be propagated in field-interacting relationship with the electron beam, and means for subjecting the electrons in said beam to a magnetic field directed substantially along the beam axis, the strength of said magnetic field being such as to produce an electron cyclotron resonance frequency substantially equivalent to the frequency of electromagnetic energy to be propagated by said fast-wave propagating circuit so that beam energy is transformed to electromagnetic wave energy at said frequency.

10. A traveling wave interaction device which comprises, a generally straight section of conductive waveguide capable of propagation of electromagnetic wave energy at a phase velocity at least equal to the velocity of light, means for injecting a beam of electrons longitudinally into said waveguide with a transverse component .of motion, and means for subjecting said electrons to a magnetic field directed substantially longitudinally of said I waveguide section and having a field strength to provide from said beam injecting means, said second waveguide 2,939,028 2/60 Cook 3153.5 X section being bent so as to collect electrons on one Wall 2,959,740 11/ 60 Adler 315-5 X thereof.

FOREIGN PATENTS References Cited by the Examiner 5 1,186,648 2/59 France.

UNITED STATES PATENTS p I 2,886,738 5/59 Tien GEORuE N. WESTBY, Przmary Exammer. 2,933,639 4/60 Lally 315-35 ARTHUR GAUSS, Examiner. 

1. A TRAVELING WAVE INTERACTION DEVICE WHICH COMPRISES, A FAST-WAVE PROPAGATING STRUCTURE, MEANS FOR INJECTING AN ELECTRON BEAM INTO SAID STRUCTURE GENERALLY LONGITUDINALLY THEREOF WITH INDIVIDUAL ELECTRONS HAVING A TRANSVERSE COMPONENT OF MOTION, AND MEANS FOR PRODUCING A MAGNETIC FIELD DIRECTED GENERALLY LONGITUDINALLY OF SAID PROPAGATING STRUCTURE AND HAVING A STRENGTH TO PROVIDE AN ELECTRON CYCLOTRON RESONANCE FREQUENCY SUBSTANTIALLY EQUAL TO THE FREQUENCY OF ELECTROMAGNETIC WAVE ENERGY PROPAGATED BY SAID WAVE-PROPAGATING STRUCTURE. 